1. Field of the Invention
This invention relates to an evaporative fuel-processing system for internal combustion engines, for purging evaporative fuel generated in the fuel tank into the intake system of the engine, and more particularly to an evaporative fuel-processing system of this kind which is capable of performing a diagnosis of abnormality of its own operation.
2. Prior Art
There has been known an evaporative fuel-processing system for an internal combustion engine having a fuel tank, which comprises a canister communicating with the fuel tank, and a purge control valve (first control valve) arranged across a purging passage extending from the canister to the intake system of the engine, wherein evaporative fuel generated in the fuel tank is temporarily stored in the canister and then suitably purged into the intake system of the engine. To determine abnormality of the thus constructed evaporative fuel-processing system, the following methods have been conventionally known:
First, an abnormality-determining method has been proposed, e.g. by Japanese PCT Provisional Patent Publication No. 4-505491, which comprises further providing pressure-detecting means for detecting pressure within the evaporative fuel-processing system, and a control valve arranged across an open-to-atmosphere passage of the canister, for selectively opening and closing the passage, and negatively pressurizing the interior of the evaporative fuel-processing system by closing the control valve and opening the purge control valve, to thereby determine whether the evaporative fuel-processing system is functioning normally, depending on whether the interior of the system has been brought into a predetermined negative pressure state.
Further, an abnormality-detecting method has been proposed by Japanese Provisional Patent Publication (Kokai) No. 4-362264, according to which the interior of the evaporative fuel-processing system is negatively pressurized, and then the purge control valve is closed, followed by determining a variation in the pressure within the evaporative fuel-processing system over a predetermined time period after the purge control valve is closed with the system negatively pressurized, to thereby determine whether or not there is an abnormality in the system, based on the determined variation.
However, according to the above proposed conventional methods, when the evaporative fuel-processing system is negatively pressurized under such a state that a large amount of evaporative fuel is adsorbed by the canister, or a large amount of evaporative fuel is generated in the fuel tank, evaporative fuel is supplied in large quantities into the intake system of the engine, so that the air-fuel ratio of a mixture supplied to the engine becomes excessively rich, which can result in degraded drivability and exhaust emission characteristics of the engine.
Further, under the above-mentioned state, the whole evaporative fuel-processing system sometimes cannot be brought into a required negatively pressurized state, which can result in an erroneous determination that the system is abnormal even when it is functioning normally.